Monday, November 14, 2011

Engineering Drums Part Four: Microphones

Microphone selection for drums is a HUGE topic that could easily fill a small book, so to keep this entry from becoming one, and in the interest of keeping things simple and practical, I will give only a very basic explanation for the types of mics typically used when tracking drums and where, on the kit, they typically go.

Dynamic microphones -- Dynamic mics function using a principle called 'electromagnetic induction.' In English this means that a conductive metal vibrating within a fixed magnetic field will generate a small current which is used to provide the mic's output signal. Since magnetic material is used to generate the magnetic field, these microphones require no external power source to function.

The two dynamic designs in use today are ribbon mics (where a thin 'ribbon' of metal is suspended directly within the magnetic field) and moving coil mics, where a plastic diaphragm is attached to a voice coil of wrapped wire which is then suspended within the magnetic field. The movement of the conductive metal (the ribbon or the voice coil) in response to sound waves generates the current, and therefore the output signal.

Because of limitations in commonly used magnetic materials these microphones tend to be less sensitive to softer dynamics as well as high frequencies (compared to higher fidelity microphone designs). The durability of the moving coil design, in particular, coupled with its relatively limited sensitivity make these mics exceptional choices for loud, bombastic signals with limited HF detail, such as kick drums, snare drums, and toms.

The physical design of ribbon microphones (an extremely thin corrugated metal ribbon suspended in an open magnetic field) make them somewhat prone to damage from plosives or other gusts of wind (which fully exert the ribbon, destroying it) as well as higher SPL sources. But the extremely low mass of the ribbon itself, coupled with the smooth character for which ribbon mics are known make them a valid choice for overheads (when you want a smooth sound) and room mics.

Condenser microphones – Condenser mics, also called 'capacitor' mics, work on an electrostatic rather than electromagnetic principle. The capsule of a condenser mic is made from two plates, one of which is fixed (called the 'backplate') and the other being moveable (called the 'diaphragm' or 'film'). Together, these two plates, along with the fixed amount of air in between them, form a capacitor (also called a condenser, hence the name, 'condenser mic'), which can store an electrical charge. A fixed DC polarization voltage is applied across this capsule, and as the air pressure (sound) causes the diaphragm to move, the distance between the two plates of the capacitor varies.  As this distance changes, so does the voltage across this capacitor.  This voltage becomes the output of the microphone.

This is a slight oversimplification of how it works (you can google 'condenser mic' if you want the whole scoop), but the bottom line is that the moveable plate of the diaphragm (most often a gold-sputtered mylar film between 3-6 microns thick) is of such a light weight that it responds well to even the lightest changes in sound pressure, making these mics sensitive to very low level signals as well as much higher frequencies than most dynamic mics. Because of their high frequency response they are great for use as overheads and on hi-hats, and given their sensitivity to low level detail, they make exceptional room / distant mics as well.

Electret condenser mics are condenser mics that have a fixed polarization charge on the capsule rather than needing an externally supplied polarizing voltage.  They still, however, require a power source for the impedance converting amplifier.

Among both externally polarized and electret condensers there are two practical variations: small diaphragm and large diaphram. Because of their even lower mass, small diaphragm condensers tend to possess an even faster response time, yielding a more neutral, lifelike response, while larger diaphragms tend to be less lifelike but a bit more opulent or larger-than-life in their presentation. For the sake of miking drums either type is perfectly appropriate.

Stay tuned, and we'll puts these mics to use...


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